1. Field of the Invention
The present invention relates to a working machine such as an excavator for actuating a working member such as a boom making use of electric power generated by a generator.
2. Discussion of the Background
In the past, in a working machine such as a hydraulic excavator, generally, a hydraulic pump is driven by making use of power of an engine mounted for self-traveling, and operating oil discharged out of the hydraulic pump is supplied to hydraulic actuators such as a swing actuator, a boom cylinder, an arm cylinder or the like to thereby drive various parts.
However, in those making use of engine power as described above, noises and exhaust gases generated from the engine adversely influence on the environments around the working field, and therefore, at present, particularly in a city area, work within a tunnel, or work in night receives a remarkable limit.
Recently, therefore, the following working machines have been developed in order to eliminate an evil caused by the operation of the engine.
A) An electrically-operated actuator is operated making use of electric power of a battery mounted on the working machine, and a hydraulic pump is driven by the electrically-operated actuator (for example, see Japanese Patent Application Laid-Open No. Hei 9 (1997)-144061 Publication).
B) There are provided a generator for generating power making use of engine power, and a condenser for suitably charging and discharging overs and shorts of power generated thereby a load of the engine is made constant to reduce consuming fuel of the engine and the quantity of exhaust gases.
However, in the battery driving type working machine in A), there is a disadvantage that as compared with the case where the engine is a power source, operating time is so short that continuous work for a long time cannot be accomplished.
On the other hand, in the so-called hybrid driving type working machine in B), there are problems as noted below.
Taking the excavator as the working machine as an example, this excavator has may working patterns such as excavation, slope finishing, leveling, hardening, scattering, crane traveling, etc. Further, loads (necessary power) greatly differ with each work, and accordingly, condensing ability required for a condenser varies.
For example, in case of excavation work, time for carrying out one operation is short, but operation of each actuator is quick, and acceleration and deceleration are frequently carried out, and therefore a peak value of load and a variation of load are great. Accordingly, a condenser must have the ability for carrying out charging and discharging operations by a large current in a comparatively short period of time. On the other hand, in case of slope finishing work or the like, time for carrying out one operation is long, but a peak value of load and a variation of load are small, and therefore, a condenser must have the ability capable of supplying power to an electrically-operated actuator little by little over a long period of time.
On the other hand, as typical examples known at present as a condenser, secondary batteries and a capacitor shown in the following Table 1 can be mentioned.
As given in this Table, the secondary batteries such as the lead battery and the nickel hydrogen battery have the characteristics that the energy density (stored energy per unit weight) is high, whereas the life of charge-discharge cycle is short, and the power density (output per unit weight) is low. Conversely, a large capacity capacitor including the electric double layer condenser has the characteristics that the power density (output per unit weight) is high and the life of charge-discharge cycle is long, whereas the energy density (stored energy per unit weight) is low.
Accordingly, where the secondary battery whose energy density is high and power density is low as described above, work for which is required to supply power little by little over a long period of time as in the slope finishing work corresponds to work for which is required a large output in a short period of time as in the excavation work despite even small weight of a condenser will well suffice, the weight of a condenser has to be made large. Conversely, where a large capacity capacitor whose power density is high and energy density is low is used, work for which is required a large output in a short period of time as in the excavation work corresponds to work carried out continuously for a long period of time as in the slope finishing work despite even a small weight of a condenser can correspond sufficiently, and therefore, the weight of a condenser has to be likewise made large.
That is, in the working machines having work which are greatly different in load characteristic from each other, even where either the secondary battery or the capacitor is applied as a condenser, either output of a condenser or stored quantity of energy results in a considerable excessive capacity. Accordingly, the cost is wasteful, and the entire working machine is difficult to be miniaturized and light-weighted.
In view of the aforementioned situation, with reducing the size and weight of the working machine, it is an object of the present invention to provide a working machine provided with a condenser capable of satisfying the condensing conditions required for various works.
For solving the aforementioned problems, the present invention employed the following constitution.
That is, the present invention provides a working machine comprising a generator, and an electrically-operated actuator in which power for actuating working members is generated by electric power generated by the generator, the working machine comprising a plurality of condensers for carrying out charging of electric power generated by the generator and supplying of electric power to the electrically-operated actuator, and a switching means for selectively switching the condensers used out of these condensers.
It is noted that the xe2x80x9celectrically-operated actuator for generating power for operating working membersxe2x80x9d includes, in addition to those directly connected to the working members to directly drive the working members, those for swing hydraulic pump included in a hydraulic circuit for driving the working members.
According to the aforesaid constitution, a condenser suitable for the load characteristic of an electrically-operated actuator in the work being carried out actually is selected out of a plurality of condensers whereby the individual condenser can satisfy the charge-discharge conditions require by the work without making it large.
For example, if a first condenser and a second condenser which is higher in output per unit weight and is lower in stored energy per unit weight than the first condenser are included, where a variation of load and a peak value of load are low but continuous charge-discharging for a long period of time is required, and where a variation of load and a peak value of load are high and charge-discharging for a short period of time and with a large current is required, the first condenser and the second condenser are selected respectively whereby even the condensers are small in size, the charge-discharging suited to the work can be carried out.
Switching of the condensers may be carried out, for example, in accordance with switching operation of a selective switch by an operator, or may be automatically carried out.
As an example of the latter, preferably, power detection means for detecting total necessary power of the electrically-operated actuator and switching control means for controlling switching of using condensers on the bass of the detected total necessary power are provided. According to this constitution, selection of proper condensers can be automatically done on the basis of power required actually.
More specifically, if a first condenser and a second condenser which is higher in output per unit weight and is lower in stored energy per unit weight than the first condenser are included as the condensers, and said switching control means is constituted such that where the detected total necessary power is within the range of power preset, the first condenser is selected, and where the detected total necessary power is larger than said range of power or smaller than said range of power, the second condenser is selected, then at the time of work for which variation of power is large and charge-discharging by a large current is required, the second condenser suited thereto is automatically selected, and at the time of work for which variation of power is small, the first condenser capable of charge-discharging for a long period of time are automatically selected, respectively.
Further, preferably, an operating member for operating the electrically-operated actuator, work discriminating means for discriminating work contents from the operating state of the operating member, and switching control means for controlling switching of using condensers according to the work contents discriminated are provided. According to this constitution, the actual work contents are discriminated on the basis of the operating contents of the operating member, and the condenser suited to the work contents is automatically selected and switched.